Porous semiconductor material

ABSTRACT

Porous semiconductor material in the form of at least partly crystalline silicon is produced with a porosity in excess of 90% determined gravimetrically, and voids, crazing and peeling are substantially not observable by scanning electron microscopy at a magnification of 7,000. The porous silicon is dried by supercritical drying. The silicon material has good luminescence properties together with good morphology and crystallinity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to porous semiconductor material such as siliconin particular, although not exclusively, to methods of producing suchmaterial and to devices incorporating it.

2. Discussion of Prior Art

In recent years great interest and considerable research and developmentactivity has been generated in response to the discovery of visibleluminescence at room temperature from porous silicon. There have been asubstantial number of publications in the scientific literature and alsopatent applications. See for example the 1992 Fall Meeting of theMaterials Research Society, the symposium being entitled"Microcrystalline Semiconductors..Materials Science and Devices", Nov.30 to Dec. 4 1992. International Patent Applicant No PCT/GB90/01901published as WO 91/09420 relates to porous silicon having luminescentproperties by virtue of its containing silicon quantum wires. Bulk (ienon-porous) silicon has very poor luminescence efficiency because it hasindirect gap band structure. Highly porous silicon containing siliconquantum wires has much greater luminescence efficiency, and theluminescence emission is of shorter wavelength. Luminescence isassociated with quantum confinement of charge carriers within quantumwires of which the porous silicon is composed.

Experience has shown that the luminescence properties of porous siliconimprove both with increasing porosity and with increasing resistivity ofthe original p-type bulk silicon starting material from which the poroussilicon is produced. However, the structural properties ofconventionally produced porous silicon degrade with both increasingporosity and increasing starting material resistivity. In Appl. Phys.Lett. Vol 60 (18), pages 2285-2287, May 4, 1992, Friedersdorf et al.discuss the influence of stress on porous silicon luminescence. Theyshow in FIG. 2 an optical micrograph of porous silicon exhibiting"cellular structure", this being the crazing and cracking of poroussilicon material. A substantial degree of delamination or peeling of theporous silicon occurs. This makes high porosity porous siliconstructurally unsuitable for luminescent device applications. It is toomechanically weak at porosities high enough to provide usefulluminescence. Experience shows that porous silicon produced inaccordance with International Patent Application No PCT/GB90/01901begins to exhibit crazing and partial disintegration at porosities above90% for bulk silicon starting material of resistivity not greater than10⁻² ohm cm (p⁺ type) and porous layer thickness of 4 μm or greater.Here the porosity is determined gravimetrically assuming no shrinkageduring production. The situation is worse with higher resistivity p typesilicon starting material (p⁻, 1 ohm cm). Here crazing and partialdisintegration occur at gravimetric porosities above 80% and layerthicknesses similar to the p⁺ equivalents.

Difficulties in producing high porosity silicon are also shown byLehmann et al., Mat. Res. Soc. Symp. Proc. Vol. 283, pages 27-32, 1993.FIG. 6 of this article demonstrates crack evolution and shrinkage duringproduction. Similar effects were also mentioned in a paper by Beale etal. J. Cryst. Growth, Vol. 73, p622 onwards 1985. This paper describeslow density porous silicon films which craze and peel during production.In a very recent paper by Grivickas et al., Thin Solid Films, Vol 235,p.234, 1993, it is stated that the thickest porous silicon films brokeoff from the substrate and disintegrated into small pieces. It isconsequently a long felt want to provide high porosity semiconductormaterial such as silicon with good structural characteristics.

Luminescent porous silicon layers having thicknesses in the range 20 μmto 80 μm are described by Badoz et al. in the Materials Research SocietySyposium Procedings, Volume 283, 1993, pages 97 to 108. Similar porousslicon material is reported by Sagnes et al. in Applied Physics Letters,Volume 62(10), 1993, pages 1155-1157. Prior art techniques for thefabrication of thick luminescent porous silicon layers are known toresult in structures in which only a top layer of the porous silicon isluminescent.

SUMMARY OF THE INVENTION

It is an object of the invention to provide porous semiconductormaterial of improved structure and a method of making it.

The present invention provides porous semiconductor material which is atleast partly crystalline, characterized in that the semiconductormaterial has a porosity in excess of 90% determined gravimetrically andin which voids, crazing and peeling are substantially indiscernible byscanning electron microscopy at a magnification of 7,000.

The invention provides the advantage that the material is of much higherstructural quality than in the prior art of high porosity crystallinesemiconductor materials.

In a preferred embodiment, the porous semiconductor material comprisesan aerocrystal of porous silicon material connected to a non-porouscrystalline silicon substrate, and is at least 80% crystalline, has athickness constant to within 10% and contains silicon quantum wires withdiameters of less than 4 nm; at least 90% by volume of the poroussilicon preferably consists of a reticulated structure of siliconquantum wires, and at least 50% of the said wires preferably havediameters of less than 4 nm. The porous silicon material is preferablyactivatable to produce visible luminescence.

The invention further provides porous semiconductor material,characterized in that the porous semiconductor material comprises anaerocrystal of greater than 90% porosity connected to a substantiallynon-porous substrate of like material and crystal structure, and thatpart of the aerocrystal has a refractive index less than 1.1.

The invention also provides porous semiconductor material which is atleast partly crystalline, characterized in that the semiconductormaterial has a porosity in excess of 90% determined gravimetrically andhas a crack density of less than 10⁸ cm⁻².

In a further aspect, the invention provides porous semiconductormaterial, characterized in that the porous semiconductor materialcomprises an aerocrystal of greater than 90% porosity connected to asubstantially non-porous substrate of like material and crystalstructure, the material being free of cracks greater than 0.1 μm inwidth.

The invention also provides porous semiconductor material which is atleast partly crystalline, characterized in that the semiconductormaterial has a porosity in excess of 90% determined gravimetrically andin which voids, crazing and peeling are substantially indiscernible byscanning electron microscopy at a magnification of 7,000 of an area ofat least 20 μm×10 μm.

In another aspect, the invention provides porous semiconductor materialwhich is at least partly crystalline, characterized in that thesemiconductor material has a porosity in excess of 90% determinedgravimetrically and is free of cracks greater than 0.1 μm in width.

In another aspect, the invention provides a luminescent deviceincorporating porous semiconductor material which is at least partlycrystalline and activatable to produce visible luminescence,characterized in that the porous semiconductor material has a porosityin excess of 90% determined gravimetrically and in which voids, crazingand peeling are substantially indiscernible by scanning electronmicroscopy at a magnification of 7,000, the device also incorporatingmeans for exciting luminescence from the porous semiconductor material.

In the luminescent device of the invention, the porous silicon materialis preferably connected to a non-porous crystalline silicon substrate,and is at least 80% crystalline, has a thickness constant to within 10%and contains silicon quantum wires with diameters less than 4 nm; atleast 90% by volume of the porous silicon preferably consists of areticulated structure of silicon quantum wires, and at least 50% of thesaid wires preferably have diameters less than 4 nm. The porous siliconmaterial is preferably activatable to produce visible luminescence.

The invention further provides a luminescent device incorporating poroussemiconductor material which is at least partly crystalline andactivatable to produce visible luminescence, characterized in that theporous semiconductor material has a porosity in excess of 90% determinedgravimetrically and in which voids, crazing and peeling aresubstantially indiscernible by scanning electron microscopy at amagnification of 7,000 of an area of at least 20 μm×10 μm, the devicealso incorporating means for exciting luminescence from the poroussemiconductor material.

In a further aspect, the invention provides a method of making poroussemiconductor material incorporating the step of producing poroussemiconductor material which is liquid-wetted and at least partlycrystalline, characterized in that the method further incorporates thestep of drying the porous semiconductor material by a supercriticaldrying process.

In the method of the invention, the step of producing poroussemiconductor material which is liquid-wetted and at least partlycrystalline preferably comprises producing porous semiconductor materialwhich is porous silicon material with a porosity in excess of 90%determined gravimetrically and which is at least partly crystalline(preferably at least 80%).

The invention further provides porous semiconductor material having aporosity of greater than 70% over a sheet thickness greater than 10 μmand which is activatable to produce visible luminescence, characterizedin that the material exhibits only a single layer structure undercross-sectional scanning electron microscopy analysis and that more than50% by thickness of the porous semiconductor material is luminescent.

The invention also provides porous semiconductor material activatable toproduce visible luminescence and having a sheet thickness greater than20 μm, characterized in that more than 80% of the porous semiconductormaterial has a luminescence efficiency of greater than 0.1%.

The invention further provides porous semiconductor material activatableto produce visible luminescence and having a sheet thickness greaterthan 10 μm, characterized in that voids, crazing and peeling aresubstantially indiscernible by scanning electron microscopy at amagnification of 7,000 and that more than 50% by thickness of the poroussemiconductor material is luminescent.

In addition, the invention provides porous semiconductor materialactivatable to produce visible luminescence, characterized in that theporous semiconductor material has a sheet thickness greater than 100 μmand that more than 50% by thickness of the porous semiconductor materialis luminescent.

The invention also provides a luminescent device incorporating poroussemiconductor material of sheet thickness greater than 10 μm and whichis activatable to produce visible luminescence, characterized in thatthe material exhibits only a single layer structure undercross-sectional scanning electron microscopy analysis and that more than50% by thickness of the porous semiconductor material is luminescent.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention might be more fully understood, examplesthereof will now be described, with reference to the accompanyingdrawings, in which:

FIG. 1 schematically shows a silicon substrate wafer and a poroussilicon layer thereon;

FIG. 2 schematically shows apparatus for supercritical drying;

FIGS. 3 to 6 are drawings produced from scanning electron microscopephotographs of porous silicon material dried in air and dried by asupercritical drying process;

FIG. 7 is a drawing of a transmission electron diffraction pattern fromporous silicon material produced in accordance with and of theinvention;

FIG. 8 shows photoluminescence spectra obtained from a porous siliconspecimen of the invention and a comparison specimen;

FIG. 9 shows measured data and theoretical model data obtained byspectroscopic ellipsometry for a porous silicon specimen of theinvention and a comparison specimen; and

FIG. 10 shows photoluminescence data from a porous silicon specimen ofthe invention.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a Czochralski-grown (cz) single crystal siliconwafer 10 350 μm in thickness is shown in section. Chain lines 11indicate that the wafer 10 is illustrated on a reduced scale. The wafer10 has a surface layer 12 of porous silicon material thereon of 4.5 μmthickness. The layer 12 has an upper surface 14 remote from the wafer10. Strictly speaking the expression "layer" is a misnomer, although itis often used in lithography.

The layer 12 was not "laid down"; it was produced electrochemically byan anodizing/etching process, ie anodization combined with etching. Thewafer 10 was of heavily doped p-type (p⁺) silicon material, with aresistivity in the range 5×10⁻³ ohm cm to 15×10⁻³ ohm cm.

The wafer 10 was anodized in what is referred to as "10% ethanoic"hydrofluoric acid (HF) solution; this produced small pores in the wafer10, which was then subjected to etching by soaking in the same solution.The etching produced pore overlap defining silicon quantum wires. Theethanoic HF solution was produced from an aqueous solution of 20% byweight of HF in water, this solution being subsequently mixed with anequal volume of ethanol. The resulting mixture is referred to as 10%ethanoic HF solution.

Anodization of the wafer 10 was carried out in the 10% ethanoic HFsolution at a current density of 50 mAcm⁻² for 3 minutes. Ananodizing/etching apparatus was employed as described in InternationalPatent Application No PCT/GB90/01901 previously mentioned. Thisanodization produced a substantially uniform gold coloured porous layer(or stratum of porous silicon material) which is 4.5±0.5 μm inthickness. The layer had a porosity (void fraction) of 85%, ie itsdensity was 15% that of non-porous crystalline silicon.

In accordance with the invention it is desired to produce a layer 12with porosity of at least 90%. Moreover, it is desired that the layer 12remain crystalline and continue to be supported by the non-poroussubstrate or wafer 10. In International Application No. PCT/GB90/01901referred to above, a procedure is disclosed in which silicon is anodizedand subsequently etched to increase porosity. However, it has been foundthat etching to a porosity in excess of 90% creates porous silicon,which, upon drying to remove the etchant, exhibits at least partialdisintegration by cracks, crazing and peeling of the porous layer fromthe substrate.

The porous layer 12 was subjected to chemical dissolution (etching) for30 minutes in 10% ethanoic HF solution (as used for anodization). Thisincreased its porosity to an average value of 95% (measured bygravimetric analysis). The layer 12 was maintained in a wet state (iewetted by the ethanoic HF etchant solution), and was transferred in thisstate to a bath of pure ethanol with etchant initially within its pores.The wafer 10 and layer 12 were cleaved under the ethanol to providereduction to a size suitable for insertion within a pressure vessel. Thecleaved wafer was then transferred to a pressure vessel of 50 ml volume.The wafer remained in an ethanol-wetted state during transfer, and thevessel itself was full of ethanol.

Referring now also to FIG. 2, there is shown in schematic form anapparatus 20 for supercritical drying of materials. The apparatus 20incorporates a CO₂ input pipe 22 connected to a pump 24 with a chilled(8° C.) pump head (not shown). The pipe 22 connects the pump 24 to a CO₂reservoir (not shown); the pump 24 is connected to a pressure vessel 26within an oven 28. The vessel 26 is connected to a back pressureregulator 30 with an outlet 32 to an effluent receiver vessel 34.

In operation of the apparatus 20, the wafer was within the pressurevessel 26. The ethanol and residual electrolyte within the pores of thelayer 12 was replaced by liquid CO₂ supplied to the pressure vessel 26by means of the pump 24. The liquid CO₂ flushed the ethanol from thelayer pores. It was pumped through the pressure vessel 26 at a flow rateof 2 cm³ per minute, a temperature of 18° C. and a pressure of 1500 psi(10.1 MPa). The level of ethanol remaining in the layer pores wasmonitored by sampling and analysing effluent from the outlet 32. Afterflushing with liquid CO₂ for 3 hours, there was less than 10 ppm ofethanol in the effluent.

The pressure vessel 26 containing the wafer 10 immersed in liquid CO₂was then subjected to increased temperature by means of the oven 28. Thetemperature within the vessel 26 was increased at 1° C. per minute to40° C., and the pressure rose gradually to 2400 psi (16.2 MPa). At thispoint the CO₂ was a supercritical fluid, ie a fluid above its criticalpoint. The pressure was then reduced to 1500 psi (10.1 MPa), andsupercritical CO₂ was flushed through the pressure vessel 26 at 40° C.for 2 hours. Maintaining the pressure vessel temperature at 40° C., theCO₂ was slowly vented from the vessel 26 over a period of 16 hoursreducing the pressure within the vessel to atmospheric. On removal fromthe vessel 26, the porous silicon layer was found to be substantiallyfree of disintegration by cracks, peeling or crazing. It had a porosity(void fraction) of 95% as determined by gravimetric analysis. Thespecimen produced in this way is referred to as UHP 23A. The siliconwafer (in a wet state and with etched porous layer) from which UHP 23Awas cleaved was dried by being allowed to stand in air as in the priorart. This wafer was then designated comparison specimen UHP 23.

The morphologies of supercritically dried specimen UHP 23A and air driedcomparison specimen UHP 23 were investigated as follows. A scanningelectron microscope (SEM) was employed to study the upper surfaces ofthese specimens, ie the equivalents of the surface 14 in FIG. 1 remotefrom the wafer 10. FIGS. 3 and 4 are drawings reproduced from suchmicrographs for specimen UHP 23 and specimen UHP 23A respectively. FIG.3 is reminiscent of fissured dried mud; it has a magnification in theregion of 1.4×10³ ; this is indicated by a line 36 having a lengthcorresponding to 10 μm in physical distance. FIG. 3 shows that air driedspecimen UHP 23 consists of regions of porous silicon such as 37(unshaded) alternating with fissures such as 34 (shaded). The fissures38 are of greater total surface area than the silicon regions 37, andare up to 8 μm in width. The silicon regions 37 are typically 2 μm to 5μm in width. In contrast, FIG. 4 is a plain featureless scene; UHP 23Aexhibited no recognisable cracks, fissures, or other morphology in theSEM photograph. In FIG. 4 a line 40 corresponds to a physical length ofapproximately 1 μm, indicating a magnification in the region of 7×10³.FIG. 4 thus illustrates an area of porous silicon of approximately 20μm×10 μm. In consequence, cracks less than 0.1 μm wide would have beenvisible if present. Specimen UHP 23A is therefore free of cracks greaterthan 0.1 μm in width.

The specimens UHP 23 and UHP 23A were cleaved perpendicular to theirupper surfaces (14 in FIG. 1), and SEM photographs were taken of thecleavage surfaces. FIG. 5 (UHP 23) and FIG. 6 (UHP 23A) were drawn fromsuch photographs; they represent magnifications of about 7×10³ asindicated by respective lines 50 and 60 corresponding to physicallengths of 1 μm. FIGS. 5 and 6 show porous silicon layers 52 and 62 uponrespective wafer substrates 54 and 64. The layer 52 of the air driedspecimen UHP 23A has substantial void or fissure regions 56, 2 μm and 5μm across, interspersed with porous silicon material 58, 8 μm across.The layer 62 of the supercritically dried specimen UHP 23A has novisible voids or fissures and is of substantially constant thickness.This shows that supercritical drying has substantially obviated theshrinkage and cracking of prior art porous silicon. From the photographsused to prepare FIGS. 4 and 6, the crack density is less than 10⁸ cm⁻².

In order to investigate the crystallinity of supercritically driedspecimen UHP 23A, a transmission electron diffraction pattern wasobtained using a 300 kV electron beam. Flakes of porous silicon wereremoved from UHP 23A and introduced into the electron diffractioninstrument. FIG. 7 was drawn from the resulting photograph. It showsinter alia a central undiffracted beam 70 with four first orderdiffraction lobes 72. The lobes 72 are characteristic of the 110orientation of crystalline silicon. Second order or (220) orientationlobes 74 are also shown. There is no evidence for the presence ofrandomly oriented crystallites, since there are no diffraction rings. Inthe photograph from which FIG. 7 was produced, there was a small amountof background intensity (not shown) in the region 76 between the centralbeam 70 and first order lobes 72. Such background can arise from anumber of phenomena, one of which is the presence of amorphous material,in this case SiO₂. The background was sufficiently weak in intensitycompared to the lobes 72 to show that the porous silicon layer 12 of UHP23A was more than 80% crystalline silicon in single crystal form; iemore than 80% of the Si atoms in the porous layer were unoxidised and oncrystal lattice sites. In fact, it is believed that more than 90% of theSi atoms were unoxidised and on silicon lattice sites. It is believedthat this porous silicon preserved the crystal structure of the originalsilicon wafer 10, since there is no other reasonable mechanism for sucha structure to arise. This is a surprising result, because thesupercritically dried specimen UHP 23A comprised quantum wires of 3 nmto 5 nm diameter in air. The interatomic spacing in silicon is 0.235 nm,so 3 nm to 5 nm diameter quantum wires have only 12 to 21 atoms across adiameter of these at least the outermost atoms will not be on crystallattice sites once they become oxidised. It is in fact surprising thatthe supercritically dried specimen UHP 23A was less than 10% oxidised,since it had been stored for one month between production and electrondiffraction measurement.

In order to compare luminescent properties of porous silicon materialproduced in accordance with the invention with that produced by a priorart procedure, two further specimens, referred to as UHP 17 and UHP 26Brespectively, were prepared. Specimen UHP 17 was produced as follows. Awafer of p⁺ silicon material with resistivity in the range 5×10⁻³ ohm cmto 15×10⁻³ ohm cm was anodized in 10% ethanoic HF solution at a currentdensity of 50 mAcm⁻² for 3 minutes. This wafer was then etched for 15minutes by being left to soak in the same solution. It was subsequentlyrinsed in pure ethanol to remove etchant, and then dried in air. As aresult of this processing, the wafer bore a porous silicon layer whichhad partly disintegrated by cracking and peeling. Detached flakes of theporous layer were imaged in a transmission electron microscope (TEM).The flakes proved to contain quantum wires with diameters in the range4.5 nm to 6 nm. A porosity value can be quoted for this layer, but itwould be of doubtful value because of the layer's partialdisintegration.

A second wafer of p⁺ silicon material with 50×10⁻³ ohm cm to 50×10⁻³ ohmcm was used to make specimen UHP 26B. The wafer was anodized for 5minutes in 10% ethanoic HF solution at a current density of 50 mAcm⁻².It was then etched for 5 minutes by being left to soak in the samesolution. Experience shows that the resulting porous silicon layer soproduced on the wafer would craze and disintegrate substantiallycompletely if dried in air. The original wafer was of higher resistivitythan that used to make UHP 17, and increase in resistivity improvesluminescence properties but also increases likelihood of disintegration.

Instead UHP 26B was dried by a supercritical drying process similar tothat described earlier with reference to FIGS. 1 and 2. The wafer wassoaked in ethanol to remove etchant from the pores of the porous siliconlayer; it was then dried in a pressure vessel of 10 cm³ capacity byreplacing the ethanol with liquid CO₂ at a temperature of 18° C. and apressure of 1500 psi (10.1 MPa).

The temperature was then raised to 40° C. over a 15 minute time intervaland flushed with supercritical CO₂ for 15 minute time interval andflushed with supercritical CO₂ for 15 minutes. A one hour interval wasemployed for depressurization of specimen to UHP 26B to atmosphericpressure. After removal from the pressure vessel, this specimenexhibited no crazing, cracking or partial disintegration. It exhibitedthe same morphology as that described for specimen UHP 23A despite therelatively more rapid depressurization.

Specimens UHP 17 and UHP 26B were tested for photoluminescenceproperties by being subjected to irradiation with 0.1 watt cm⁻² of argonion laser radiation of 458 nm wavelength. This was carried out in air atambient temperature. Specimen UHP 17 had undergone storage in air atambient temperature subsequent to formation and prior to irradiation.The photoluminescence spectra are shown in FIG. 8, graphs (a) and (b)being those of Specimens UHP 17 and UHP 26B respectively. The ordinateaxis is photoluminescence intensity (arbitrary units) and the abscissaaxis is wavelength (nm). Graph (a) has been displaced vertically toavoid overlapping spectra; the graphs both go to zero at 550 nm (atwhich a relative shift has been introduced), and graph (a) has undergonea tenfold (×10) ordinate scale expansion or relative magnificationcompared to graph (b). This can be appreciated from the much greaternoise level of graph (a) relative to that of graph (b).

The photoluminescence emission band of graph (a) is wider than that ofgraph (b). Moreover, graph (b) has a peak photoluminescence emissioncentred at about 757 nm, compared with about 778 nm for graph (a). Graph(b) is therefore blue-shifted slightly relative to graph (a), and hashigher peak intensity having regard to the relative magnification ofgraph (a) previously mentioned. The integrated photoluminescenceintensity (peak intensity multiplied by full width at half height) ofgraph (b) is six times that of graph (a). These differences arecollectively consistent with specimen UHP 26B incorporating smallerquantum wires than specimen UHP 17, giving a higher degree of quantumconfinement, and having higher quantum wire density. Calculations showthat specimen UHP 26B exhibits a photoluminescence efficiency of greaterthan 0.1%. Furthermore, FIG. 8 is evidence that supercritical dryingdoes not merely preserve the luminescence properties of porous silicon,but also provides enhancement of those properties. The combination ofspecimen UHP 26B and the activating Argon ion laser constitutes aluminescent device.

An example of the invention, specimen UHP 23C cut from the same wafer asUHP 23A, was used to investigate the morphology of material of theinvention. Transmission electron microscopy (TEM) was used to studyspecimen UHP 23C; this specimen proved to contain silicon quantum wireswith diameters less than 4 nm, and it consisted of a three dimensionalreticulated structure of silicon quantum wires at least 50% of whichwere less than 4 nm in diameter. At least 90% of the silicon in thisstructure was in the form of silicon quantum wires.

The invention provides inter alia an electrochemical process forproducing porous crystalline silicon material supported by and attachedto a bulk (ie non-porous) silicon substrate, the porous silicon beinginitially in a wetted state and dried by a supercritical drying process.The invention provides crystalline porous silicon with luminescenceproperties; the porous silicon is attached to and is an extension of thecrystal structure of non-porous crystalline silicon material from whichthe porous silicon was produced by removal of silicon material. Theporous silicon is a three dimensional branched network of quantum wiresin which the luminescence properties arise from quantum confinement ofcharge carriers. Luminescence properties of porous silicon are improvedby increase in quantum confinement, which requires reduction in quantumwire diameter and therefore increase in porosity. At least in p⁺silicon, these properties are also improved by increased resistivity ofthe bulk silicon wafer used to produce the porous silicon material.

Porosity data quoted above was obtained for specimens UHP 23A and othersby a gravimetric procedure described in International Application No.PCT/GB90/01901 previously mentioned. This procedure involves determiningthe weight of silicon lost in anodization and etching and calculating anaverage density for the porous silicon assuming it retains itspre-anodization shape. The porous silicon density is then divided by thedensity of bulk silicon, and the result is subtracted from unity toprovide a porosity figure. The validity of this depends on the poroussilicon preserving its shape. If there is shrinkage during drying, theactual porosity will be lower than that indicated by gravimetric data.

Ellipsometric spectroscopy was employed to determine the thickness ofthe air dried comparison specimen UHP 23 and the supercritically driedspecimen UHP 23A. The results are shown in FIG. 9. Ellipsometry is aknown technique for optically characterising surfaces and surface filmsor layers. It involves determining the amplitude ratio (tan ψ) and phasedifference (cos Δ) of light beams reflected from a surface under testand p-polarized and s-polarised respectively in relation to thatsurface. This is carried out over a range of wavelengths. Subsequentlythe data is fitted to a theoretical model, in which the surface layer'spseudo-refractive indices and thickness are parameters. Porosity isobtainable from comparison with the optical properties of non-porous(bulk) crystalline silicon. See for example Pickering et al., AppliedSurface Science 63 (1993) pages 22-26.

In FIG. 9, the ellipsometric amplitude ratio ψ (Psi) in degrees isplotted against photon energy (eV) for UHP 23 (air dried) and UHP 23A(supercritically dried). This is shown in each case both for measuredellipsometry data and for calculations from a theoretical model forminga best fit to such data. The upper right of the drawing gives a key tothe graphs shown. The theoretical model employed is a multilayereffective medium model of a kind known in ellipsometry. Graphs 90(measured data) and 92 (theoretical model) were obtained formsupercritically dried specimen UHP 23A shown in FIGS. 4 and 6. Theoscillatory behaviour is due to interference fringes, and indicates goodoptical quality. The theoretical model is a reasonable fit; itoverestimates fringe amplitude, probably due porosity gradient and/ornon-ideal interfaces. It indicates a thickness of the porous siliconlayer of 4.98±0.05 μm, together with a porosity (percentage void)varying from 97% (surface remote from substrate) to 92% (adjacent tosubstrate). This is consistent with etching decreasing with penetrationdepth, which is normal. UHP 23A therefore has a thickness of about 5 μmand an average porosity of about 95% determined ellipsometrically. Theporosity figure of 97% corresponds to the outer or upper 0.25 μmthickness of the porous silicon 62 in FIG. 6. This porosity isconsistent with an average density of 0.09 g/cm³ and a refractive indexless than 1.10.

Graphs 94 (measured data) and 96 (theoretical model) were obtained forthe air dried comparison specimen UHP 23. They do not show interferencefringes, indicating that coherency with the substrate has been lost andthat optical quality is poor. The measured data can be fitted with anaverage porosity of 83% assuming a free standing porous layer; this isconsistent with cross-sectional SEM results (see FIG. 5) showing thatthe porous layer had at least partially lifted off (become detached)from the bulk silicon substrate. The data also indicated a reverse ornegative porosity gradient of 80% at the outer surfaces increasing to85% adjacent to the substrate; this is consistent with collapse of atleast part of the porous structure.

The difference between the ellipsometric porosity results for UHP 23 andUHP 23A indicates that the former experienced a shrinkage over 10%relative to the latter. Such shrinkage can be inferred from FIG. 5, inwhich the porous silicon material contains voids 54 and has a concaveupper surface consistent with shrinking during drying. FIG. 6 howeverdemonstrates that such shrinkage is not detectable in supercriticallydried specimen UHP 23A. From FIG. 6 it can be seen that the thickness ofspecimen UHP 23A is constant to within 5% over a distance of 20 μmmeasured parallel to the surface of the silicon substrate 64. Thisthickness is 4.0±0.2 μm. Over the whole of specimen UHP 23A (area 2cm²), porous silicon thickness variation was less than 10%. Thiscompares with a variation in thickness of air dried specimen UHP 23which approaches 100%; ie the thickness varies between about 4 μm andless than 0.1 μm due to voids 56 in the porous silicon material of thisspecimen. The voids 56 extend virtually down to the substrate 54, andthe near 100% thickness variation occurs over distances of only a few μmbetween adjacent voids 56.

Supercritical drying is known per se for the production of aerogels,these being structures which are over 90% porous and the pores are airfilled. Tewari et al. describe the production of transparent silicaaerogels in Materials Letters, Vol. 3, No. 9, 10 pages 363 to 367, July1985. The silica aerogel is produced from a solution by a sol-gelprocess; the process involves hydrolysis and polycondensation of siliconalkoxides in alcohol, which produces an alcogel (ie alcohol-filledpores). The aerogel is produced by replacing the alcohol in the pores byair. This is done by supercritical drying. The silica aerogels arenon-crystalline and are free standing, ie there is no supportingsubstrate. They are insulators not semiconductors, and Tewari et al. donot disclose luminescence properties.

Supercritical drying for the production of silica aerogels is alsodisclosed by Rangarajan and Lira, J. Supercritical Fluids, Vol 4, No. 1,pages 1 to 7, 1991. The original work in this field was carried out byKistler in 1932, and is reported in J. Phys. Chem 1932, 36, 52.Published Japanese Patent Application Number JP 890266463 describes amethod of drying semiconductor substrates using supercritical dryingtechniques to give increased yield through the avoidance of attachingforeign matter to the substrate.

Semiconductor processing is a high technology field with considerableresearch and development activity in many countries. Semiconductorprocessing is directed to layers or strata supported by bulk(non-porous) wafer substrate. Layers grown epitaxially on a substrateare extensions of the substrate's crystal structure, ie a substrate andsemiconductor layers thereon from a single crystal. The layers are ofmicroscopic thickness, typically a few μm in thickness or less down to afraction of a μm. A porous silicon "layer" is a skeleton structureformed by etching to remove material from a silicon wafer to which theskeleton is attached. It is referred to in this specification, as anaerocrystal or aerocrystal network, and is an extension of a non-porouscrystalline substrate. Pores in the porous silicon terminate at thewafer. Aerogels are formed by condensation from solution, not from anoriginal non-porous solid phase material. An aerogel therefore has nosubstrate of such original material, and all its surfaces are porous.Aerogels are amorphous, ie non-crystalline; they are of macroscopicdimensions, mm or cm in thickness, as opposed to μm thicknesses forsemiconductor layers.

In a textbook, "Chemical Processing of Advanced Materials", pp 19onwards, Ed. Hench and West, 1992, John Wiley and Sons, Hrubesh et al.describe the production of very high porosity silica aerogels. Theyobserve that such aerogels are hydrophobic if produced by directextraction of solvent, and are stable in atmosphere. However, silicaaerogels are hydrophilic when produced by first exchanging the solventwith carbon dioxide and supercritically drying. They shrink badly whenexposed to air. This has surprisingly been found not to be the case forsupercritically dried porous silicon. It is also surprising that thehigh pressures involved in supercritical drying have not produced anyserious etching effects associated with the solvent being displaced fromporous silicon. In this regard it is observed that alcohols in poroussilicon are associated with heat evolution indicating etching. This isreported by Canham and Groszek in J. Appl. Phys. Vol 72 (4), pages1558-1564, Aug. 15, 1992.

The supercritical drying process may also be used to fabricate thickluminescent porous silicon layers which are of the order of 200 μm inthickness. Such layers having a porosity of greater than 70% averagedover the depth of the porous layer have been produced and theirluminescence properties characterized.

Referring now to FIG. 10, there is shown in graphical form a plot ofluminescence intensity in arbitrary units versus depth for across-section of a thick luminescent porous silicon layer. The thickluminescent porous silicon layer was fabricated by the following method.A wafer of silicon material heavily doped with arsenic to form a n⁺wafer, with a resistivity in the range 5×10⁻³ ohm cm to 10×10⁻³ ohm cmwas used to produce the very thick porous layer. The wafer was anodizedin a 20% ethanoic HF solution, comprising a 1:1 ethanol:40 wt % HFsolution, at 50 mAcm⁻² for 90 minutes. The wafer was then subjected to achemical etch in the same solution for 2 hours. Following the etchingprocess, the wafer was removed from the HF solution and immersed inisopropyl alcohol (IPA) whilst still wet. The wafer was kept immersed inthe IPA for 18 hours prior to commencement of the supercritical dryingprocess. The supercritical drying was carried out in a similar manner tothat described earlier in relation to FIG. 2 except that that IPA ratherthan ethanol was used as the intermediate solvent. Segments of the waferwere dried in a 50 cm³ pressure vessel which was flushed with liquid CO₂at a temperature of 18° C. and a pressure of 10.1 MPa with a flow rateof 5 cm³ min⁻¹ for 80 minutes to remove most of the IPA. The temperaturewas then raised at a rate of 2° C. min⁻¹ to 40° C. and the sampleflushed with supercritical CO₂ for 50 minutes. The pressure vessel wasthen de-pressurized at 40° C. over a period of 10 minutes. The wafersegment had a porous silicon layer approximately 200 μm thick with aporosity of greater than 70% throughout the entire depth of the layer.The porosity was measured using a gravimetric technique. Aftersupercritical drying, the wafer segment was stored in ambient air for 47days prior to luminescence measurements being performed.

FIG. 10 was obtained using measurement techniques as described by Bealeet al. in Materials Research Society Symposium Proceedings, Volume 283,1993, pages 377-382. A cross-section of the porous silicon layer wasilluminated using a 442 nm HeCd laser and the intensity of the resultingvisible photoluminescence was measured as a function of position acrossthe porous silicon layer. The extent of the porous silicon region isindicated by an arrow 100, with the porous silicon region having top andbottom surfaces corresponding to doffed lines 102 and 104 respectively.FIG. 10 shows that photoluminescence was achieved throughout the poroussilicon layer, with the luminescence intensity decreasing withincreasing depth. The efficiency of the luminescence was measured forthe top porous silicon surface, i.e. the surface furthest from thesilicon wafer, corresponding to dotted line 102 at a depth of 0 μm inFIG. 10. The measured external power efficiency was 0.54%. Using thisvalue for luminescence efficiency at the porous silicon top surface, itis possible to calibrate FIG. 10. The luminescence efficiency fallsbelow 0.3% at a depth indicated by a chain line 105, indicating that theporous silicon has a luminescence efficiency of more than 0.3% over morethan 100 μm, or 50% of the porous silicon thickness. The luminescenceefficiency falls below 0.1% at a depth indicated by a chain line 106,indicating that the porous silicon has a luminescence efficiency ofgreater than 0.1% over more than 160 μm, or 80% of the porous siliconthickness. The porous silicon layer therefore has more than 80% of itsthickness exhibiting good luminescence properties. The combination ofthe thick porous silicon layer and the illuminating laser comprises aluminescent device. The wafer was examined in an SEM and was found tohave good structural integrity, with no evidence of cracking of theporous silicon layer being observed in a sample measuring 2 cm by 4 cm.

Thick photoluminescent porous silicon with a thickness of 32±5 μm andgood structural integrity, showing no evidence of cracking underscanning electron microscopy has also been fabricated from lightly dopedp-type (p⁻) silicon. A cz p⁻ silicon wafer of resistivity in the range1-3 ohm cm was anodized in a 20% ethanoic HF solution for 90 minutes at20 mAcm⁻². The anodized wafer was then immersed in IPA whilst still wet.The wafer was then dried supercritically in the same manner as for the200 μm thick sample described previously. Visible photoluminescence wasobserved across the entire porous silicon layer under cross-sectionalflood illumination with 442 nm radiation. When illuminated with 325 nmultraviolet radiation, the porous silicon luminesced with a peakwavelength of 575 nm. The efficiency of the photoluminescence wasmeasured to be more than 1% extending over a thickness of greater than20 μm.

Thick porous silicon layers have been observed previously. N. Ookubo etal. in Material Science and Engineering B20 (1993) pp 324 to 327reported the production of porous silicon films having a thickness of upto 94 μm. However, the films produced by Ookubo et al. have a layeredstructure, with only a top layer of thickness 6-8 μm being described asvisibly photoluminescent. The top surface of the Ookubo films are shownas being cracked into cells with a lateral dimension of approximately 10μm. In another paper by Ookubo in J. Appl. Phys 74 (10), Nov. 15, 1993,pp 6375 to 6382, describing a two-layer structure of approximately 8 μmin thickness, the layer structure is attributed to the process ofremoving the electrolyte after anodization. Grivickas et al. in theirpaper referenced earlier also observed a layered structure to theirporous silicon films under scanning electron microscopy. These filmswere up to 50 μm thick and are described as being weaklyphotoluminescent but again showed cracking. It seems probable that thelayered structure is a result of the top layer having higher porositywhich collapses when the sample is dried. It is desirable to be able toform luminescent porous silicon layers of thickness greater than 10 μmwhich do not have a layered structure. It is also desirable to formluminescent porous silicon layers of thickness greater than 10 μm whichdo not exhibit cracking. Neither the 200 μm thick n⁺ porous siliconlayer nor the 32 μm thick p⁻ porous silicon layer exhibited a layerstructure under cross-sectional scanning electron microscopy. Also,neither the 200 μm thick n⁺ porous silicon layer nor the 32 μm thick p⁻porous silicon layer exhibited voids, crazing or peeling duringexamination by scanning electron microscopy at a magnification of 7,000.

Whereas the invention is particularly relevant to porous silicon, it isalso relevant to other porous semiconductor materials such as porousgallium arsenide.

I claim:
 1. Porous semiconductor material which is at least partlycrystalline, wherein the semiconductor material has a porosity in excessof 90% determined gravimetrically and in which voids, crazing andpeeling are substantially not observable by scanning electron microscopyat a magnification of 7,000, wherein the porous semiconductor materialis porous silicon material.
 2. Material according to claim 1, whereinthe material is at least 80% crystalline.
 3. Material according to claim3, wherein the material is connected to a non-porous silicon substrate.4. Material according to claim 1, wherein the material is connected to anon-porous silicon substrate.
 5. Material according to claim 1, whereinthe material contains silicon quantum wires with diameters less than 4nm.
 6. Material according to claim 5, wherein at least 90% by volume ofthe material consists of a reticulated structure of silicon quantumwires and at least 50% of the said wires have diameters less than 4 nm.7. Material according to claim 6, wherein the material is at least 80%crystalline.
 8. Material according to claim 1, wherein the material isin the form of an aerocrystal connected to a non-porous crystallinesilicon substrate wherein the aerocrystal and the substrate have thesame crystal structure.
 9. Material according to claim 1, wherein thematerial is activatable to produce visible luminescence.
 10. Materialaccording to claim 2, wherein the material is in the form of anaerocrystal connected to a non-porous crystalline substrate of the samesemiconductor material.
 11. Porous semiconductor material, wherein theporous semiconductor material comprises an aerocrystal of greater than90% porosity connected to a substantially non-porous substrate of thesame material and crystal structure, and that part of the aerocrystalhas a refractive index less than 1.1, said material comprising silicone.12. Porous semiconductor material which is at least partly crystalline,wherein the semiconductor material has a porosity in excess of 90%determined gravimetrically and has a crack density of less than 10⁸cm⁻², said material comprising silicone.
 13. Porous semiconductormaterial wherein the porous semiconductor material comprises anaerocrystal of greater than 90% porosity connected to a substantiallynon-porous substrate of the same material and crystal structure, thematerial being free of cracks greater than 0.1 μm in width, saidmaterial comprising silicone.
 14. Porous semiconductor material which isat least partly crystalline, wherein the semiconductor material has aporosity in excess of 90% determined gravimetrically and in which voids(56), crazing and peeling are substantially not observable by scanningelectron microscopy at a magnification of 7,000 of an area of at least20 μm×10 μm, said material comprising silicone.
 15. Porous semiconductormaterial which is at least partly crystalline, wherein the semiconductormaterial has a porosity in excess of 90% determined gravimetrically andis free of cracks greater than 0.1 μm in width, said material comprisingsilicone.
 16. A luminescent device incorporating porous semiconductormaterial which is at least partly crystalline and activatable to producevisible luminescence, wherein the porous semiconductor material (12) hasa porosity in excess of 90% determined gravimetrically and in whichvoids, crazing and peeling are substantially not observable by scanningelectron microscopy at a magnification of 7,000, the device alsoincorporating means for exciting luminescence from the poroussemiconductor material, said material comprising silicone.
 17. A deviceaccording to claim 16, wherein the porous silicon material is at least80% crystalline.
 18. A device according to claim 16, wherein the poroussilicon material contains silicon quantum wires with diameters less than4 nm.
 19. A device according to claim 18, wherein at least 90% by volumeof the porous silicon material consists of a reticulated structure ofsilicon quantum wires and at least 50% of the said wires have diametersless than 4 nm.
 20. A device according to claim 16, wherein the poroussilicon material is connected to a non-porous crystalline siliconsubstrate.
 21. A luminescent device incorporating porous semiconductormaterial which is at least partly crystalline and activatable to producevisible luminescence, wherein the porous semiconductor material has aporosity in excess of 90% determined gravimetrically and in which voids,crazing and peeling are substantially not observable by scanningelectron microscopy at a magnification of 7,000 of an area of at least20 μm×10 μm, the device also incorporating means for excitingluminescence from the porous semiconductor material, said materialcomprising silicone.
 22. A method of making porous semiconductormaterial incorporating the step of producing porous semiconductormaterial which is liquid-wetted and at least partly crystalline, themethod further incorporates the step of drying the porous semiconductormaterial by a supercritical drying process, wherein the step ofproducing porous semiconductor material which is liquid-wetted and atleast partly crystalline involves producing porous semiconductormaterial which is porous silicon material with a porosity in excess of90% determined gravimetrically.
 23. A method according to claim 22,wherein the step of producing porous semiconductor material which isliquid-wetted and at least partly crystalline involves anelectrochemical process to produce porous silicon material.
 24. A methodaccording to claim 23, wherein after drying the porous silicon materialhas at least 80% crystallinity, porosity in excess of 90% determinedgravimetrically, and voids, crazing and peeling are not observable byscanning electron microscopy at a magnification of 7,000.
 25. A methodaccording to claim 24, wherein after drying at least 90% by volume ofthe porous silicon material consists of a reticulated structure ofsilicon quantum wires and at least 50% of the said wires have diametersless than 4 nm.
 26. A method according to claims 22, wherein thesupercritical drying process involves replacement of liquid in the poresof the porous semiconductor material by liquid CO₂ and a subsequentchange of the CO₂ phase from liquid to gas.
 27. A method of makingporous semiconductor material according to claim 22, wherein afterdrying the porous semiconductor material is activatable to producevisible luminescence.
 28. A method according to claim 22, wherein theporous semiconductor material has a sheet thickness greater than 10 μm.29. A method according to claim 28, wherein the material has a sheetthickness greater than 100 μm.
 30. A method according to claim 28,wherein the material has a sheet thickness in the range 20 μm to 200 μm.31. Porous semiconductor material produced by the method of claim 22.32. A method of making porous semiconductor material incorporating thestep of producing porous semiconductor material which is liquid-wettedand at least partly crystalline, the method further incorporates thestep of drying the porous semiconductor material by a supercriticaldrying process,wherein the step of producing porous semiconductormaterial which is liquid-wetted and at least partly crystalline involvesproducing porous semiconductor material which is porous silicon materialwith a porosity in excess of 90% determined gravimetrically, wherein thestep of producing porous semiconductor material which is liquid-wettedand at least partly crystalline involves an electrochemical process toproduce porous silicon material, wherein the step of producing poroussemiconductor material which is liquid-wetted and at least partlycrystalline involves an anodization/etching process to produce poroussilicon material connected to a substantially non-porous crystallinesilicon substrate.
 33. A method according to claim 32, wherein theanodization/etching process comprises anodizing a substantiallynon-porous crystalline silicon substrate to produce porous and at leastpartly crystalline silicon material connected thereto, and etching theporous silicon material to increase its porosity, etching beingterminated with the porous silicon material in a liquid-wetted state fordrying by the supercritical drying process.